Power supply monitoring for an implantable device

ABSTRACT

A method and an apparatus for determining a time period remaining in a useful life of an energy storage device in an implantable medical device. The method may include measuring a voltage of the energy storage device to produce a measured voltage, and comparing the measured voltage to a transition voltage. While the measured voltage is greater than or equal to the transition voltage, the time period remaining in the energy storage device&#39;s useful life is approximated based upon a function of charge depleted. While the measured voltage is less than the transition voltage, the time period remaining in the energy storage device&#39;s useful life is approximated based upon a higher order polynomial function of the measured voltage. The transition voltage corresponds to a predetermined point on a energy storage device voltage depletion curve representing the voltage across the energy storage device over time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. patent application Ser. No. 11/588,798, filed on Oct. 27, 2006, nowissued as U.S. Pat. No. 7,711,426, and entitled “Power Supply MonitoringFor An Implantable Medical Device,” which is a continuation-in-part ofand claims priority to U.S. patent application Ser. No. 11/341,978,filed on Jan. 27, 2006, now issued as U.S. Pat. No. 7,769,455, andentitled “Power Supply Monitoring For An Implantable Device” all ofwhich are incorporated herein by reference.

BACKGROUND

1. Technical Field

This disclosure relates generally to implantable medical devices, andmore particularly to monitoring power consumption.

2. Background Information

There have been many improvements over the last several decades inmedical treatments for disorders of the nervous system, such as epilepsyand other motor disorders, and abnormal neural discharge disorders. Oneof the more recently available treatments involves the application of anelectrical signal to reduce various symptoms or effects caused by suchneural disorders. For example, electrical signals have been successfullyapplied at strategic locations in the human body to provide variousbenefits, including reducing occurrences of seizures and/or improving orameliorating other conditions. A particular example of such a treatmentregimen involves applying an electrical signal to the vagus nerve of thehuman body to reduce or eliminate epileptic seizures, as described inU.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is herebyincorporated by reference in its entirety in this specification.Electrical stimulation of the vagus nerve may be provided by implantingan electrical device underneath the skin of a patient and electricallystimulating tissue, organ(s) or nerves of the patient. The system mayoperate without a detection system if the patient has been diagnosedwith epilepsy, and may periodically apply a prophylactic series ofelectrical pulses to the vagus (or other cranial) nerve intermittentlythroughout the day, or over another predetermined time interval.

Typically, implantable medical devices (IMDs) involving the delivery ofelectrical pulses to, or sensing electrical activity of, body tissues,such as pacemakers (heart tissue) and vagus nerve stimulators (nervetissue), comprise a pulse generator for generating the electrical pulsesand a lead assembly coupled at its proximal end to the pulse generatorterminals and at its distal end to one or more electrodes in contactwith the body tissue to be stimulated. One of the key components of suchIMDs is the power supply (e.g., a battery), which may or may not berechargeable. In many cases surgery is required to replace an exhaustedbattery. Even rechargeable batteries eventually may need replacement. Toprovide adequate warning of impending depletion of the battery andsubsequent degradation of the operation of the IMD, various warningsignals or indicators may be established and monitored.

Generally, battery-powered IMDs base warning signals or indicators onbattery voltage and/or battery impedance measurements. One problemassociated with these methodologies is that, for many batterychemistries, these measured battery characteristics do not havemonotonically-changing values with respect to remaining charge. Forexample, lithium/carbon monofluoride (Li/CFx) cells commonly used inneurostimulators and other IMDs have a relatively flat voltage dischargecurve for the majority of their charge life, and present status of thebattery cannot be accurately and unambiguously determined from ameasured battery characteristic.

More specifically, in LiCFx batteries, the battery voltage remainsrelatively constant for approximately 90% of its useful life and thenreaches a point where the battery changes from a linear region ofapproximately zero slope to an approximately linear or downwardlycurving region of negative slope. Thus, during the last 10% of batterylife (when battery voltage versus battery depletion enters the secondrange), an added term in the projection equation that incorporatesbattery voltage may improve the accuracy of the projection to thebattery's depletion.

Another problem associated with impedance-based methodologies is thevariability of current consumption for a specific device's programmedtherapy or circuitry. This variability, combined with the uncertainty ofthe battery's present status prior to depletion, hinders reliableestimation of the anticipated time until reaching the end of thebattery's useful life. For scheduling purposes, it is desirable to havea constantly available and reliable estimate over all therapeuticparameter ranges and operation settings of the time until the devicewill reach the end of its useful life, and provide an indication whenthat time reaches a specific value or range.

The present disclosure is directed to overcoming, or at least reducing,the effects of, one or more of the problems set forth above.

BRIEF SUMMARY

In accordance with various embodiments, a method is provided fordetermining a time period remaining in a useful life of an energystorage device in an IMD. The method may include measuring a voltage ofthe energy storage device to provide a measured voltage, and comparingthe measured voltage to a transition voltage. If the measured voltage isgreater than or equal to the transition voltage, the time periodremaining in the energy storage device's useful life is approximatedbased upon a function of charge depleted. If, on the other hand, themeasured voltage is less than the transition voltage, the time periodremaining in the energy storage device's useful life is approximatedbased upon a higher order polynomial function of the measured voltage.The transition voltage corresponds to a predetermined point on a batteryvoltage depletion curve representing the voltage across the energystorage device over time.

In accordance with various embodiments, an implantable medical device isprovided. The IMD includes an energy storage device that provides powerfor the IMD, a stimulation unit operatively coupled to the energystorage device that provides an electrical signal, and a controlleroperatively coupled to the stimulation unit and the energy storagedevice. The controller may include a charge depletion determinationunit, a voltage determination unit, and a useful life determinationunit. The charge depletion determination unit determines an electricalcharge depleted by the energy storage device during operation of theIMD. The voltage determination unit determines whether a measuredvoltage across the energy storage device is greater than or equal to atransition voltage. The useful life determination unit determines a timeperiod remaining in the energy storage device's useful life basedupon 1) a function of the electrical charge depleted when the measuredvoltage is greater than or equal to the transition voltage, and 2) ahigher order polynomial function of the measured voltage when themeasured voltage is less than the transition voltage. The transitionvoltage corresponds to a predetermined point on the battery voltagedepletion curve.

In accordance with various embodiments, a system is provided fordetermining remaining useful life of a battery in an IMD. The systemincludes an IMD and an external monitoring device. The IMD delivers anelectrical signal to a patient and communicates with an externalmonitoring device. The IMD includes the battery that powers the IMD, anda controller operatively coupled to the battery that determines anelectrical charge depleted by the battery and a voltage across thebattery. The external monitoring device determines a remaining usefullife of the battery based upon (1) the electrical charge depleted if thevoltage across the battery is greater than or equal to a transitionvoltage and (2) a higher order polynomial function of the voltage acrossthe battery if the voltage across the battery is less than thetransition voltage. The external monitoring device also displays anindication of the remaining useful life of a battery of the IMD.

In an embodiment, the voltage across the energy storage device (i.e.,battery) may be characterized by a higher order polynomial function whenthe measured voltage is less than the transition voltage. Accordingly,the time remaining in such embodiments may be calculated according tothe following polynomial function: T_(EOS)=%remaining*(C_(initial)/I_(avg)) wherein percentage (%)remaining={(−b+SQRT[b²−4a(c−V_(bat))])/2a}.

In still another embodiment, extreme temperatures may adversely affectthe calculation of time remaining. Therefore, the measured Voltage maybe compensated for temperature according to the following equation:V_(compensated)=V_(measurement)+(K×(T_(baseline)−T_(measurement))); andthe time remaining may be more accurately calculated using theV_(compensated) value as the battery voltage.

In yet another embodiment, the transition voltage may vary according tothe average current consumption rate I_(avg). By incorporating thevariable transition voltage in the determination of time remaining,where V_(knee) is the greater of W volts and X−LOG(I_(avg)/Y))/Z, whereW is a variable or constant voltage, X is a voltage greater than Wvolts, I_(avg) is the average current consumption rate, and Y and Z arepredetermined coefficients, time remaining may be more accuratelydetermined.

In still another embodiment, during early stages of battery life,battery impedance may be measured at an artificially high level,impacting the measured voltage and therefore the calculation of Timeremaining. Thus, the measured voltage may be corrected with an impedancecorrection factor for use in determining the time period remaining inthe energy storage device's useful life. The impedance correction factormay be a constant, or the impedance correction factor may vary withI_(avg), such that the Voltage used to calculate timeremaining=V_(measured)+[R_(const)×I_(avg)], where V_(measured) is themeasured voltage, R_(const) is the constant impedance correction factorand I_(avg) is the variable average current consumption rate.

The preferred embodiments described herein do not limit the scope ofthis disclosure.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, implant companies may refer to a components or groups ofcomponents by different names. This document does not intend todistinguish between components or groups thereof that differ in name butnot function. In the following discussion and in the claims, the terms“including” and “comprising” are used in an open-ended fashion, and thusshould be interpreted to mean “including, but not limited to . . . .”

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 is a stylized diagram of an implantable medical device suitablefor use in the present disclosure implanted into a patient's body and anexternal programmer;

FIG. 2 is a block diagram of an implantable medical device and anexternal unit that communicates with the implantable medical device, inaccordance with one illustrative embodiment of the present disclosure;

FIGS. 3A and 3B are graphical representations of a battery voltagedepletion curve typical of an energy storage device or battery, inaccordance with illustrative embodiments of the present disclosure;

FIG. 4 is a flowchart representation of a method of providing a warningsignal relating to a power supply of the implantable medical device, inaccordance with one illustrative embodiment of the present disclosure;

FIG. 5 is a flowchart representation of a method of performing acalibration of a charge depletion tabulation, in accordance with oneillustrative embodiment of the present disclosure;

FIG. 6 is a more detailed flowchart illustrating a method of performingthe charge depletion calculation indicated in FIG. 4, in accordance withone illustrative embodiment of the present disclosure;

FIG. 7 is a more detailed flowchart illustrating a method of performingan end-of-service (EOS) and/or an elective replacement indication (ERI)determination, as indicated in FIG. 4, in accordance with oneillustrative embodiment of the present disclosure;

FIG. 8 is an illustrative graphical representation of battery capacityestimation for a battery having a voltage depletion curve described by apolynomial equation in the region after the transition voltageV_(knee).;

FIG. 9 is an illustrative graphical representation comparison of lowload and high load coefficients for the polynomial equation of FIG. 8,including an intermediate polynomial that represents intermediatecoefficients achieved with rounded averages;

FIG. 10 is an illustrative graphical representation of how V_(knee)varies as a function of I_(avg) and program settings in accordance withone embodiment of the present disclosure;

FIG. 11 is a graphical representation of the approximately linearrelationship between the temperature and the number of volts necessaryto correct V_(bat) for temperature in accordance with one illustrativeembodiment of the present disclosure; and

FIG. 12 is a detailed flowchart illustrating an alternative method ofperforming an end-of-service (EOS) and/or an elective replacementindication (ERI) determination, as indicated in FIG. 4, in accordancewith one illustrative embodiment of the present disclosure.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the disclosure to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

DETAILED DESCRIPTION

Illustrative embodiments of the disclosure are described herein. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. In the development of any such actualembodiment, numerous implementation-specific decisions must be made toachieve the design-specific goals, which will vary from oneimplementation to another. It will be appreciated that such adevelopment effort, while possibly complex and time-consuming, wouldnevertheless be a routine undertaking for persons of ordinary skill inthe art having the benefit of this disclosure.

Embodiments of the present disclosure provide methods and apparatus formonitoring and/or estimating the time remaining until the generation ofan elective replacement indicator or until the end of service of thebattery of an implantable medical device (IMD). Estimating battery lifemay be based upon 1) estimated future charge depletion and actual pastcharge depletion while the battery is operating in a first part of itsuseful life and 2) on the measured voltage in a second part of itsuseful life. Specifically, when the battery operates at a voltagegreater than or equal to a voltage associated with a transition point ofnon-linearity in the battery voltage depletion curve shown in FIG. 3A(referred to herein as the knee), estimating the battery life is basedon the charge depleted, an example of which is disclosed in U.S. patentapplication Ser. No. 10/902,221, filed Jul. 28, 2004, incorporatedherein by reference in its entirety. When the battery operates at avoltage less than a voltage associated with the knee, estimating thebattery life may be determined as a function of measured voltage acrossthe battery, rather than the total charge depleted. Embodiments of thepresent disclosure provide for the generation an elective replacementindicator (ERI) signal to provide a warning for performing an electricaldiagnostic operation upon the IMD. This electrical diagnostic operationmay include replacing an electrical component in the IMD, performingadditional evaluation (s) of the operation of the IMD, replacing orrecharging a power source of the IMD, and the like. A more detaileddescription of an IMD suitable for use in the present disclosure isprovided in various figures and the accompanying description below.

Generally, IMDs contain a power storage device (e.g., a battery) toprovide power for the operations of the IMD. Embodiments of the presentdisclosure determine an estimated usable life remaining in the batteryunit based upon where in the battery's useful life it is operating.Embodiments of the present disclosure may be performed in a standalonemanner within the IMD itself or in conjunction with an external devicein communication with the IMD. Utilizing embodiments of the presentdisclosure, an end of service (EOS) signal or an elective replacementindicator (ERI) signal may be provided, indicating that the IMD is at ornear termination of operations and/or the battery power has reached alevel at which replacement should be considered to avoid interruption orloss of therapy to the patient. An ERI signal may indicate that anelectrical device component, such as a battery, has reached a pointwhere replacement or recharging is recommended. An EOS signal mayprovide an indication that the operation of the implanted device is at,or near, termination and delivery of the intended therapy can no longerbe guaranteed. ERI and EOS are commonly used indicators of the presentstatus of an IMD battery. ERI is intended to be a warning signal of animpending EOS indication, providing sufficient time (e.g., several weeksor months) in typical applications to schedule and perform thereplacement or recharging.

FIG. 1 illustrates a generator 110 having main body 112 comprising acase or shell 121 with a connector 114 for connecting to leads 122. Thegenerator 110 is implanted in the patient's chest in a pocket or cavityformed by the implanting surgeon just below the skin, similar to theimplantation procedure for a pacemaker pulse generator. A stimulatingnerve electrode assembly 125, preferably comprising an electrode pair,is conductively connected to the distal end of an insulated electricallyconductive lead assembly 122, which preferably comprises a pair of leadwires (one wire for each electrode of an electrode pair). Lead assembly122 is attached at its proximal end to the connector 114 on case 121.The electrode assembly is surgically coupled to a vagus nerve 127 in thepatient's neck. The electrode assembly 125 preferably comprises abipolar stimulating electrode pair, such as the electrode pair describedin U.S. Pat. No. 4,573,481 issued Mar. 4, 1986 to Bullara. Persons ofskill in the art will appreciate that many electrode designs could beused in the present disclosure. The two electrodes are preferablywrapped about the vagus nerve, and the electrode assembly 125 ispreferably secured to the nerve 127 by a spiral anchoring tether such asthat disclosed in U.S. Pat. No. 4,979,511 issued Dec. 25, 1990 to ReeseS. Terry, Jr. and commonly owned by the assignee of the instantapplication. Lead assembly 122 may be secured, while retaining theability to flex with movement of the chest and neck, by a sutureconnection to nearby tissue.

The pulse generator 110 may be controlled or programmed with an externaldevice 150 (e.g., a computer) and a programming wand 155 to facilitateradio frequency (RF) communication between the external device 150 andthe pulse generator 110. The wand 155 and software permit noninvasivecommunication with the generator 110 after the latter is implanted.

FIG. 2 illustrates one embodiment of IMD 110 for performingneurostimulation in accordance with embodiments of the presentdisclosure. In one embodiment, the implantable medical device 110comprises a battery unit 210, a power-source controller 220, astimulation controller 230, a power regulation unit 240, a stimulationunit 250, a memory unit 280 and a communication unit 260. It will berecognized that one or more of the blocks 210-280 (which may also bereferred to as modules) may comprise hardware, firmware, software, orany combination thereof. The memory unit 280 may be used for storingvarious program codes, starting data, and the like. The battery unit 210comprises a power-source battery that may be rechargeable ornon-rechargeable. The battery unit 210 provides power for the operationof the IMD 110, including electronic operations and the stimulationfunction. The battery unit 210, in one embodiment, may be alithium/thionyl chloride cell or a lithium/carbon monofluoride cell. Theterminals of the battery unit 210 are preferably electrically connectedto an input side of the power-source controller 220 and the powerregulation unit 240.

The power-source controller 220 preferably comprises circuitry forcontrolling and monitoring the flow of electrical power to variouselectronic and stimulation-delivery portions of the IMD 110 (such as themodules 230, 240, 250, 260, and 280 illustrated in FIG. 2). Moreparticularly, the power-source controller 220 is capable of monitoringthe power consumption or charge depletion of the implantable medicaldevice 110, measuring the voltage across the battery unit 210, andgenerating ERI and/or EOS signals. The power-source controller 220comprises an active charge-depletion unit 222, an inactivecharge-depletion unit 224, and a voltage determination unit 225, and anoptional calculation unit 226.

The active charge-depletion unit 222 is capable of calculating thecharge depletion rate of the implantable medical device 110 while theimplantable medical device 110 operates in one or more “active states.”Each active state may have an associated charge depletion rate that isthe same as or different from at least one other active state. Theactive state of the implantable medical device 110 refers to a period oftime during which a stimulation pulse is delivered by the implantablemedical device 110 to body tissue of the patient according to a firstset of stimulation parameters. Other active states may include states inwhich other activities are occurring, such as status checks and/orupdates, or stimulation periods according to a second set of stimulationparameters different from the first set of stimulation parameters.

The inactive charge-depletion unit 224 is capable of calculating thecharge depletion rate of the implantable medical device 110 duringinactive states. Inactive states may comprise various states ofinactivity, such as sleep modes, wait modes, and the like.

The voltage determination unit 225 is capable of measuring or receivingthe voltage across the battery unit 210 at any given point in time tocompare the measured voltage to a transition voltage associated with thetransition point of non-linearity on the battery voltage depletion curve(shown in FIG. 3A), or the knee. The comparison may be used to determinewhether to use the charge depleted, or a function of the measuredvoltage to determine the time remaining until generation of an ERI ortime remaining until EOS. In an embodiment, the voltage determinationunit 225 determines whether the voltage across the battery unit 210 isless than the transition voltage associated with the knee. If thebattery unit 210 has a measured voltage across it that is greater thanor equal to the voltage associated with the knee on the battery voltagedepletion curve, the time to ERI or EOS is calculated according to thecharge depleted. This calculation may be performed in an externalERI/EOS calculation unit 272 of the external unit 150 after the measuredvoltage and accumulated charge are communicated from the IMD 110 to theexternal unit 150. Alternatively, this calculation may be performed inan optional calculation unit 226 of the power-source controller 220, andthe result may be communicated from the IMD 110 to the external unit150.

If the battery unit 210 has a measured voltage that is less than thevoltage associated with the knee, the time to ERI or EOS is calculatedbased on a function of the measured voltage. This calculation may beperformed in an external ERI/EOS calculation unit 272 of the externalunit 150 after the measured voltage and accumulated charge arecommunicated from the IMD 110 to the external unit 150. Alternatively,this calculation may be performed in an optional calculation unit 226 ofthe power-source controller 220, and the result may be communicated fromthe IMD 110 to the external unit 150. One or more of the activecharge-depletion unit 222, the inactive charge-depletion unit 224, andthe voltage determination unit 225, the optional calculation unit 226 ofthe power-source controller 220 and/or the external ERI/EOS calculationunit 272 may be hardware, software, firmware, and/or any combinationthereof.

The power regulation unit 240 is capable of regulating the powerdelivered by the battery unit 210 to particular modules of the IMD 110according to their needs and functions. The power regulation unit 240may perform a voltage conversion to provide appropriate voltages and/orcurrents for the operation of the modules. The power regulation unit 240may comprise hardware, software, firmware, and/or any combinationthereof. The communication unit 260 is capable of providing transmissionand reception of electronic signals to and from an external unit 150.

The external unit 150 may be a device that is capable of programmingvarious modules and stimulation parameters of the IMD 110. In oneembodiment, the external unit 150 is a computer system capable ofelectronic communications, programming, and executing a data-acquisitionprogram, preferably a handheld computer or PDA. The external unit 150 ispreferably controlled by a healthcare provider such as a physician, at abase station in, for example, a doctor's office. The external unit 150may be used to download various parameters and program software into theIMD 110 for programming the operation of the implantable device. Theexternal unit 150 may also receive and upload various status conditionsand other data from the IMD 110. The communication unit 260 may comprisehardware, software, firmware, and/or any combination thereof.Communications between the external unit 150 and the communication unit260 may occur via a wireless or other type of communication, illustratedgenerally by line 275 in FIG. 2.

In an embodiment, the external unit 150 comprises an ERI/EOS calculationunit 272 capable of performing calculations to generate an ERI signaland/or an EOS signal. If the battery unit 210 has a measured voltageacross it that is greater than or equal to the voltage associated withthe knee on the battery voltage depletion curve (as shown in FIG. 3A),the time to ERI or EOS is calculated according to a function of thetotal charge depleted, and if the battery unit 210 has a measuredvoltage that is less than the voltage associated with the knee, the timeto ERI or EOS is calculated based on a function of the measured voltage.These calculations may be performed either in an optional calculationunit 226 of the power-source controller 220 or in an ERI/EOS calculationunit 272 of the external unit 150. The external ERI/EOS calculation unit272 may additionally be capable of receiving calculations performed inthe optional calculation unit 226 in the IMD 110 that are communicatedto the external unit 150. The external ERI/EOS calculation unit 272 maybe hardware, software, firmware, and/or any combination thereof.

Stimulation controller 230 defines the stimulation pulses to bedelivered to the nerve tissue according to parameters and waveforms thatmay be programmed into the IMD 110 using the external unit 150. Thestimulation controller 230 controls the operation of the stimulationunit 250, which generates the stimulation pulses according to theparameters defined by the controller 230 and in one embodiment providesthese pulses to the lead assembly 122 and electrode assembly 125.Various stimulation signals provided by the implantable medical device110 may vary widely across a range of parameters. The Stimulationcontroller 230 may comprise hardware, software, firmware, and/or anycombination thereof.

FIG. 3A provides a graphical representation of a battery voltagedepletion curve in accordance with an embodiment of the presentdisclosure. The battery voltage depletion curve shown in FIG. 3A ischaracterized by a first region 300 represented by an approximatelylinear function having a slope of approximately zero, and a secondregion 302 represented by an approximately linear function having anegative slope. The first region 300 is defined between the beginning ofthe life of the battery at time 304 and a time associated with the knee306. The time associated with the knee 306 corresponds to a transitionpoint along the battery voltage depletion curve between the first region300 and the second region 302. The time associated with the knee 306corresponds to a voltage referred to herein as V_(knee) 310. The secondregion 302 is defined between the time associated with the knee 306 anda time associated with the approximate end of service of the battery308, a point in time when the battery is no longer effective forpowering the IMD 110 for stimulation. The time associated with theapproximate end of service of the battery 308 corresponds to a voltagereferred to herein as V_(EOS) 312.

FIG. 3B provides a graphical representation of a battery voltagedepletion curve in accordance with another embodiment of the presentdisclosure. The battery voltage depletion curve shown in FIG. 3B ischaracterized by a first region 300 represented by an approximatelylinear function having a slope of approximately zero, and a secondregion 302 represented by a non-linear approximation comprised of ahigher order polynomial with a downwardly curving slope. In a stillfurther embodiment of the present disclosure, the battery voltagedepletion curve may be approximated by a non-linear approximationcomprised of a higher order polynomial. In such a non-linearapproximation, V_(knee) 310 may be selected from various data pointsalong the curve associated with a point in time when the voltage beginsto sharply drop.

In one embodiment of the disclosure, the IMD 110 determines EOS and ERIvalues when the battery unit 210 is in the part of its useful batterylife (the first region 300) having voltage greater than or equal to thevoltage associated with the knee by using a known initial battery charge(C_(initial)) and a predetermined EOS battery charge (C_(EOS))indicative of the end of useful battery service, together with thecharge actually depleted (C_(d)) by the IMD (calculated from the presentusage rates for idle and stimulation periods (r_(i) and r_(s)), and thelength of the respective idle and stimulation periods), to calculate fora desired time point how much useful charge remains on the battery(C_(r)) until the EOS charge is reached, and how long at projectedpresent usage rates the device can operate until EOS or ERI. Once thecharge actually depleted by operation of the device (C_(d)) is known,the present usage rates are then applied to the remaining useful chargeC_(r) to determine the time remaining until EOS and/or ERI.

In one embodiment of the disclosure, the IMD 110 determines EOS and ERIvalues when the battery unit 210 is in the part of its useful batterylife (the second region 302) having voltage less than the voltageassociated with the knee by using the measured value for voltage acrossthe battery unit 210, a value for the voltage across the battery unit210 at the time of EOS (which may be previously set, determined, orprogrammed into the IMD), a value for the voltage across the batteryunit 210 at the knee (which may be previously set, determined, orprogrammed into the IMD), a constant representing a percentage ofbattery capacity commonly remaining in the battery when the batteryvoltage is equal to the voltage associated with the knee (which may bepreviously set or determined or programmed into the IMD), the knowninitial battery charge C_(initial), and present usage rates.

In at least some embodiments, EOS and ERI determinations are madewithout measurements or calculations of internal battery impedance.Instead, in the portion of the battery's useful life (the first region300) with battery voltage greater than or equal to the voltageassociated with the knee, the device maintains a precise record of thecurrent used during idle and stimulation periods, and subtracts thecharge represented by the current used from the total available batterycharge to determine the charge remaining on the battery. Alternatively,in the portion of the battery's useful life (the second region 302) withthe battery voltage less than the voltage associated with the knee, thedevice bases the time remaining on the measured voltage across thebattery, known or predetermined voltage characteristics, and presentusage rates.

Consistent with the foregoing, FIG. 4 provides a flowchart depiction ofa method for determining the remaining time to EOS and/or ERI based onknown or determined IMD characteristics such as battery charge orvoltage and present usage rates, depending on where in its useful lifethe battery is operating. In one embodiment, the present usage of theIMD 110 is calibrated during manufacture (step 410). Current drawn bythe IMD from the battery is defined as electrical charge per unit time.The total charge depleted from the battery as a result of the operationsof the IMD may be determined by multiplying each distinct current rateused by the IMD by its respective time used. In one embodiment, as partof the calibration, during manufacturing, a power supply capable ofgenerating a known voltage and a meter capable of measuring a knowncurrent may be used to characterize the power consumption or currentdepletion of the implantable medical device 110 during its stimulationand idle modes. The power consumption data thus obtained is preferablystored in a memory of the IMD.

Once the charge usage characteristics of the IMD are known, the batterymay be subsequently installed into the implantable medical device 110for operation and thereafter a record of power consumed by theimplantable medical device 110 is maintained. In a particularembodiment, the calibration step 410 involves calibration of presentusage for idle periods (r_(i)) and stimulation periods (r_(s)) of thedevice. Current may thus be used as a proxy value for electrical chargedepletion, and the calibration step allows a precise determination ofthe amount of electrical charge used by the device after implantation.As used herein, the terms “depletion rate,” “consumption rate,” and“usage rate” may be used interchangeably, unless otherwise indicated,and refer to the rate at which electrical charge is depleted from thebattery. However, as noted above, current may be used as a proxy forelectrical charge, and where this is the case, current rates r_(i) andr_(s) may also be referred to as “present usage,” “current rate,”“current consumption,” “charge depletion,” “depletion rate” or similarterms.

As previously noted, the IMD 110 has a number of settings and parameters(e.g., current, pulse width, frequency, and on-time/off-time) that canbe changed to alter the stimulation delivered to the patient. Thesechanges result in different usage rates by the IMD 110. In addition,circuit variations from device to device may also result in differentpresent usage rates for the same operation. Calculations and estimationsare preferably performed during the manufacturing process in order tocalibrate accurately and precisely the present usage rates of the IMD110 under a variety of stimulation parameters and operating conditions.A calibration of the present usage rates and a determination of thecharge present on the battery at the time of implant allow a moreaccurate assessment of actual and predicted charge depletion after theIMD 110 is implanted. The initial charge on the battery may include asafety factor, i.e., the charge may be a “minimum charge” that allbatteries are certain to possess, even though many individual batteriesmay have a significantly greater charge. Nothing herein precludes adetermination of a unique initial charge for each individual battery.However, it will be recognized that such individual determinations maynot be economically feasible. A more detailed illustration anddescription of the step (410) of calibrating present usage andinitializing the battery charge for the implantable medical device 110is provided in FIG. 5 and the accompanying description below.

After calibrating the present usage characteristics of the IMD 110, theIMD may be implanted and subsequently a charge depletion calculation isperformed (step 420). In an embodiment, the charge depletion calculationis performed periodically, or in an alternative embodiment, the chargedepletion calculation is performed each time the external unit 150 is incommunication with the IMD 110. This calculation may be performed by theIMD itself, the external unit 150, or by both, and includes determiningthe actual electrical charge depleted from the battery 210 andestimating current consumption (i.e., depletion rates), which may,depending on where the battery is operating within its useful life, beused to calculate an elective replacement indication (ERI) and/or an endof service (EOS) signal (step 430). A more detailed illustration anddescription of the step 420 of calculating the electrical chargedepleted is provided in FIG. 6 and the accompanying description below.In step 425, the voltage across the battery unit 210 is measured todetermine whether the battery unit 210 is operating at a voltage greaterthan, equal to, or less than a voltage associated with the knee, ortransition point of non-linearity in the battery voltage depletion curve(as shown in FIG. 3A). In step 430 an estimated time until an electivereplacement indication will be generated and/or the estimated time untilthe end of service are calculated based on whether the battery unit 210is operating at a voltage greater than, equal to, or less than a voltageassociated with the knee. A more detailed description and illustrationof the step 430 of calculating the time to ERI and/or EOS is provided inFIG. 7 and the accompanying description below.

Referring now to FIG. 5, a flowchart diagram is provided depicting ingreater detail the step 410 (FIG. 4) of calibrating and initializing theIMD 110 during manufacturing. In one embodiment, the current rates forthe IMD 110 during stimulation are calibrated (block 510). Duringmanufacturing, several different combinations of measurements may becalibrated. More specifically, measurements of charge depletion relatingto different types of pulses (i.e., pulses having different stimulationparameters) are calibrated to ensure that present usage measurements forthe IMD are accurate over a wide range of stimulation parameters. Inother words, various pulses having a range of current amplitudes, pulsewidths, frequencies, duty cycles and/or lead impedances into which thepulses are delivered are used to calibrate the measurement of presentusage during stimulation to establish a baseline of the measurement ofcharge depletion for various types of pulses. All operational variablesrelating to or affecting the present usage rates of the IMD may beconsidered.

More particularly, during manufacture of the IMD 110, severalcombinations of data points relating to various current rates resultingfrom various combinations of pulse parameters are used in one embodimentto generate an approximately linear equation that relates various pulseparameters to current rate, which may then be used to determine chargedepletion when the battery unit 210 is operating at a voltage greaterthan or equal to the voltage associated with the knee. For example, fora first stimulation, pulses of a certain frequency are provided and fora second stimulation, the frequency of the pulses used may be doubled.Therefore, the estimated present usage rate for the second stimulationmay be estimated to be approximately double that of the powerconsumption or charge depleted due to the first stimulation. As anotherexample, a first stimulation may be of a first pulse width and a secondstimulation may be of a pulse width that is double that of the width ofthe first pulse. Therefore, a relationship between the pulse width tothe current consumption of the second pulse may be estimated to beapproximately double that of the present usage rate of the first pulse.In one embodiment, a graph may be generated using the various types ofstimulation versus the current consumption associated with thatstimulation.

As yet another example, a first stimulation pulse may have a firstcurrent amplitude and a second stimulation may have a current amplitudethat is double that of the first stimulation pulse. Therefore, thecurrent consumption of the second stimulation pulse may be estimated tobe approximately double that of the current consumption of the firststimulation pulse. The power consumption is directly proportional to thecurrent consumption. Therefore, a relationship of a pulse parameter topresent usage rate may be estimated or measured such that aninterpolation may be performed at a later time based upon the linearrelationship developed during the calibration of the power consumptionduring stimulation. It may be appreciated that the relationships of somepulse parameters to present usage rate may not be a simple linearrelationship, depending upon such pulse characteristics as the type ofpulse decay (i.e., square wave, exponential decay), for example.Nevertheless, calibration of present usage rate for various pulseparameters may be performed by routine calculation or experiment forpersons of skill in the art having the benefit of the presentdisclosure.

Referring again to FIG. 5, present usage during an idle (i.e.,non-stimulating) period is calibrated in step 520. From the idle currentconsumption and the stimulation current consumption calibration, theoverall current consumption may be modeled based upon programmedsettings. It should be noted that while the disclosure as shown in thedrawings describes a device having two present usage patterns associatedwith an idle period and a stimulation period, such a two-stateembodiment is described solely for clarity, and more complex embodimentsare possible involving a third state such as, by way of non-limitingexample, a present usage rate associated with electrical sensing of thelead electrodes, which may be defined by a third current rate r₃.Four-state or even higher state embodiments are possible, although wherethe differences in present usage rates are small, or where a particularpresent usage rate comprises only a tiny fraction of the overall time ofthe device, the complexity required to implement and monitor the timesuch current rates are actually used by the device may render the deviceimpractical. These multi-state embodiments may be implemented ifdesired, however, and remain within the scope and spirit of the presentdisclosure.

Using the calibration of present usage during stimulation periods (step510) and idle periods (step 520), a calculation may optionally be madeto initialize the charge depleted, if any, during manufacturingoperations, such as the charge depleted during testing of the deviceafter assembly (block 530). In a preferred embodiment, all of thecalibrations are performed with a calibrated current source device, andnot a battery, and in this case there is no charge depletion duringmanufacturing operations. In another embodiment, the amount of chargedepleted during manufacturing may be small, in which case theinitialization procedure may also be omitted. The calibration and/orinitialization steps of FIG. 5 allow the IMD 110, via power-sourcecontroller 220, to maintain a running tally of how much charge has beendepleted from the device. When the battery unit 210 is first insertedinto the implantable medical device 110, the charge depleted isgenerally initialized to zero so that a running tabulation may beginfrom zero for maintaining a running tally of the charge depleted fromthe battery over the life of the implantable medical device 110. In oneembodiment, the charge depleted tally is incremented throughout theoperating life of the device and at any point the running tally may besubtracted from the known initial charge of the battery to determine theremaining charge. In an alternative embodiment, the charge depletedtally could be initialized to the value of the battery initial chargeand the tally decremented throughout the device operation and directlyused as the remaining charge. In either implementation, informationrelating to the baseline charge remaining on the battery at the end ofmanufacturing may be retained to calculate the estimated time to EOS orERI when the battery is operating at a voltage greater than or equal tothe voltage associated with the knee.

Turning now to FIG. 6, a flowchart depiction of the step 420 ofcalculating charge depleted by the device is provided in greater detail.For simplicity, only the two-current state of a single idle period and asingle stimulation period is shown. Embodiments having additionalpresent usage rates are included in the present disclosure. The IMD 110may determine a current depletion rate r_(i) for idle periods (block610). The rate is preferably stored in memory. In one embodiment, thedetermination is made by the IMD 110 after implantation. In a preferredembodiment, the idle current depletion rate may be a rate determinedduring manufacturing (i.e., a rate calibrated in step 520) and stored inthe memory 280. An idle period is defined as a time period when theimplantable medical device 110 is not performing active stimulation,i.e., is not delivering a stimulation pulse to the electrodes. Variouselectronic functions, such as tabulation and calculation of numbers orexecution of various software algorithms within the IMD 110 may takeplace during the idle period.

As noted, the current rate r_(i) during idle periods 610 may bepredetermined during the manufacturing process (step 520) and mayinclude various considerations, such as the power consumption of theoperation of various electronics in the implantable medical device 110,even though no active stimulation may be taking place during that timeperiod. However, since the implantable medical device 110 may beoccasionally reprogrammed while still implanted inside a patient's body,the number and duration of idle periods may vary according to the dutycycle and frequency of the stimulation pulses. Therefore, the IMD 110(e.g., via the power source controller 220 in the device) may maintain arunning tabulation of the idle periods, and for each idle period acertain amount of charge depleted during the idle period (i.e., offtime) is tabulated and stored in memory 280 (step 620).

It will be appreciated that the depleted charge may be obtained in anumber of different ways, each within the scope of the presentdisclosure. Specifically, the total time of all idle periods sinceimplantation, initialization, or since a previous idle power depletioncalculation, may be maintained as a running total idle time in memory,or alternatively a running tally of charge depleted during idle periodsmay be maintained. While these values are different numerically, theyare directly related by simple equations as discussed more fullyhereinafter. At an update time, the total idle time may be periodicallyaccessed and multiplied by the idle period present usage rate todetermine the total power depleted during idle periods sinceimplantation, initialization, or the previous calculation.

The IMD 110 may also maintain in memory 280 a tabulation of presentusage rates (i.e., charge depletion) for a wide range of stimulationsettings (step 630). In another embodiment, theoretical charge depletioncalculations relating to particular types of stimulation may be providedto the IMD 110. The stimulation parameter settings may then be used bythe device to maintain a running tabulation of the charge depletedduring stimulation periods using a present usage rate r_(s) calculatedfrom the pulse width, pulse amplitude, pulse frequency, and otherparameters which may impact the present usage rate. This method providesspecific present usage rates for a variety of stimulation parametersettings and lead impedances without requiring the storage of presentusage rates for all possible stimulation parameter settings and leadimpedances.

In one embodiment, the charge depleted may be stored in micro-ampseconds; however, various other measurement units may be utilized. Inone embodiment, the IMD 110 itself may be capable of calculating thepresent usage rate for a particular combination of programmed outputsettings based upon a known relationship between present usage rates anddifferent combinations of programmed settings. The relationship may thenbe used to interpolate a particular present usage rate for a particularcombination of programmed output settings. However, in order to reducethe computation load on the device, some or all of these calculations,including the interpolation, are preferably performed by an externalprogrammer 150. Therefore, upon programming or performing routinemaintenance of the implantable medical device 110, the external unit 150may perform the calculations to determine the present usage rate duringfuture stimulation cycles based upon the settings implemented during theprogramming or maintenance operation.

For example, if the stimulation for a particular patient is set to aparticular pulse width, the external device 150 may factor in thecalibration data and determine a present usage rate for a particular setof stimulation settings. Therefore, for each stimulation period, thecharge that is depleted is tabulated for the stimulation period(“on-time”) by multiplying the stimulation time by the present usagerate and a running tabulation is maintained (block 640). For example, ifthe predetermined present usage rate for each second of stimulation at aparticular combination of parameter settings is 100 microamps, and thestimulation is 30 seconds long, a calculation is made by multiplying the30 second time period for the stimulation, by the 100 microamps toarrive at 3000 micro amp seconds of charge consumed, which is then addedto a running charge consumption tally.

As illustrated in FIG. 6, the sum of the tabulations of the chargedepleted for the idle period (off-time or inactive period; step 620) andthe charge depleted for the stimulation period (on-time or activeperiod; step 640) are added to arrive at a total charge depleted by theIMD 110 (block 650). It will be appreciated that the sum of idle periodand stimulation charge depletion may occur at the conclusion of one ormore cycles of idle period and stimulation period, or continuouslythroughout idle periods and stimulation periods. Occasionally during theoperational life of the IMD 110, various stimulation parameters may bechanged to provide different types of stimulation. However, utilizingthe steps described herein, a running tally (or a periodically updatedtally) of the charge depletion is maintained, such that even when thestimulation settings change, the device maintains a substantiallyaccurate reflection of the actual charge that has been depleted by theIMD 110, and future depletion calculations are based on the depletionrate for the newly programmed settings.

The memory 280 may store the results of the charge calculations (step660). The data stored may include both the present usage rates for idleand stimulation periods of the IMD 110, as well as the total chargedepleted. This data may be utilized by the IMD 110 and/or external unit150 to determine various aspects of the device, including the timeremaining until an ERI is generated, or the time remaining until EOS.

The calculations associated with steps 620, 640 and 650 may be expressedmathematically. In particular, the total charge available from thebattery C_(tot) after it is placed in the IMD 110 may be represented asthe difference between an initial battery charge C_(initial) and the EOSbattery charge C_(EOS), as expressed in Equation 1.C_(tot)=C_(initial)−C_(EOS)  (Eq. 1).

The charge depleted by the IMD 110 during idle periods C_(i) (step 620)may be expressed as the idle period present usage rate r_(i) multipliedby the total duration of all idle periods Δt_(i) according to equation2.C_(i) =r _(i) ×ΣΔt ^(i)  (Eq. 2).

Where multiple idle rates are present, the above equation will be solvedfor each idle present usage rate and the results summed to obtain C_(i).Similarly, the charge depleted during stimulation periods C_(s) (step640) may be expressed as the stimulation period present usage rate r_(s)multiplied by the total duration of all stimulation periods Δt_(s)according to equation 3.C_(s) =r _(s) ×ΣΔt _(s)  (Eq. 3).

Again, where multiple stimulation rates are used the equation will besolved for each stimulation rate and the results summed. The totalcharge depleted C_(d) is the sum of C_(i) and C_(s), as shown inequation 4.C_(d)=C_(i)+C_(s)  (Eq. 4).

Finally, the charge remaining until EOS (C_(r)) at any arbitrary pointin time is the difference between the total energy or charge availableC_(tot) and the charge actually depleted from the battery C_(d) at thatsame point in time, as expressed in equation 5 (step 650).C_(r)=C_(tot)−C_(d)  (Eq. 5).

This may be accomplished by counters that record the amount of time thedevice uses the idle present usage rate(s) and the stimulation presentusage rate(s), respectively, which are then multiplied by the applicablepresent usage rate to obtain the total consumed charge during the idleand stimulation periods. Alternatively, separate registers may directlymaintain a running tally of the charge depleted during stimulationperiods and idle periods, respectively.

Turning now to FIG. 7, a more detailed flow chart depicting thecalculation of the time to the end of service (EOS) and/or electivereplacement indicator (ERI) signals, as indicated in step 430 of FIG. 4,is illustrated. The IMD 110 is programmed for delivering to the patientelectrical pulses having predetermined parameters (step 710).Programming the stimulation settings may be performed duringmanufacturing and/or by a healthcare provider when the external unit 150communicates with the IMD 110. Occasionally, medical personnel maydetermine that an alteration of one or more of the stimulationparameters is desirable. Implementation of such changes may easily beaccomplished to optimize the therapy delivered by the IMD.Alternatively, as part of a routine diagnostic process, a predeterminedchange to the stimulation settings may be performed. Additionally, theIMD 110 may have multiple sets of stimulation parameters stored inmemory and may switch between the different stimulation modesrepresented by those parameters at preset times or at the occurrence ofcertain physiological events. When a change in one or more stimulationparameter settings is implemented (whether by programming or accessingdata from memory), the IMD 110 and/or the external unit 150 maydetermine an updated stimulation period present usage rate r_(s)associated with the new parameter settings, and subsequent updates tothe total charge consumed will be based upon the new stimulation periodpresent usage rate (step 720). The rates may either be stored in memoryor calculated from an equation by interpolation among known currentrates for known parameter settings, as previously described. It is alsopossible that changes to the software or firmware of the device couldchange the idle period depletion rate, in which event a new idle periodpresent usage rate r_(i) may also be calculated and reflected insubsequent calculations of total charge depleted (step 720).

Because the duty cycle (on-time to off-time ratio) is also a programmedparameter, the present disclosure allows both the idle period presentusage rate (r_(i)) and the stimulation period present usage rate (r_(s))to be combined into a single rate for purposes of projecting futureenergy or charge depletion and calculating a time to EOS and/or ERI.This rate represents the total present usage rate (r_(t)) of the device(step 725). Following updates to the stimulation and/or idle periodpresent usage rates r_(s) and r_(i), the updated rates are then used tocalculate a new total charge remaining C_(r), by a method substantiallyas shown in FIG. 6 and previously described (step 730).

Additionally, the voltage across the battery unit is measured (step740). In an embodiment, the voltage across the battery may be measuredperiodically while in an alternative embodiment, the voltage across thebattery is measured when the IMD 110 is in communication with theexternal unit 150. The measured voltage across the battery in step 740and the charge depleted in step 730 are stored in a memory in the IMD110 and/or transmitted from the IMD 110 to the external unit 150 forstorage in a memory in the external unit 150 (step 750).

Once the total charge remaining and measured voltage across the batteryunit are retrieved from memory, the remaining time to an activation ofan EOS is calculated depending on where in the battery's useful life itis operating. A comparison of the measured battery voltage and thevoltage associated with the knee is performed to see whether themeasured battery voltage is greater than or equal to the voltageassociated with the knee (step 760).

When the measured voltage across the battery unit is greater than orequal to the voltage associated with the knee, the time remaining iscalculated by dividing the remaining charge by the total depletion rateas shown in Equation 6 (Step 770).t=(C_(r))/(r _(r))  (Eq. 6).

Equation 6 may be expressed in more discrete terms as shown in Equation7.t=(C_(initial)−C_(EOS)−C_(d))/I_(avg)  (Eq. 7).

Wherein I_(avg) is the current consumption rate (i.e., estimated futuredepletion rate) based upon the IMD's present settings. Alternatively,I_(avg) may be based on proposed settings, as desired, to estimate thetime until EOS impact of altering one or more of the settings. Generallyspeaking, I_(avg) is not a constant, but varies from one generator tothe next, and also varies with the program settings.

When the measured voltage across the battery unit is less than thevoltage associated with the knee, the time remaining is calculated bydividing the product of the difference between the measured voltage(V_(bat)) and a voltage associated with the EOS (V_(EOS) 312), theinitial charge (C_(initial)), and a percentage of battery capacitytypically remaining at the knee (% knee) by the product of the averagecurrent (I_(avg)) and the different between the voltage associated withthe knee (V_(knee) 310) and the voltage associated with the EOS (V_(EOS)312), as shown in Equation 8 (Step 775).t={(V_(bat)−V_(EOS))×%_(knee)×C_(initial)}/{I_(avg)×(V_(knee)×V_(EOS))}  (Eq.8).

Applying Equation 8 regardless of where the battery is along the batteryvoltage depletion curve results in accuracy only in the last negativeslope region of battery life, and inaccuracy in the range of voltagesgreater than the voltage associated with the knee, which is why it isadvantageous to use Equation 7 at measured voltages greater than orequal to the voltage associated with the knee and Equation 8 at measuredvoltages less than the voltage associated with the knee. Equations 7 and8 may be combined, for clarity's sake, into a single, non-linearequation, as shown in Equation 9.

$\begin{matrix}{\begin{matrix}\; & \begin{matrix}\left( {C_{initial} - C_{EOS} - C_{d}} \right) \\I_{avg}\end{matrix} & {x \geq V_{knee}} \\{{T_{EOS}(x)} =} & \; & \; \\\; & \begin{matrix}{\left\{ {V_{bat} - V_{EOS}} \right) \times \%_{knee} \times C_{initial}} \\{I_{avg} \times \left( {V_{knee} - V_{EOS}} \right)}\end{matrix} & {x < V_{knee}}\end{matrix}.} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

Applying Equation 9 assures accuracy in the ranges of voltages greaterthan, equal to, and less than the voltage associated with the transitionpoint of non-linearity in the battery voltage depletion curve.

The time remaining calculated in Step 770 or 775 may be stored in amemory of the IMD 110, or alternatively may be stored and/or transmittedto the external unit 150 (step 780). At a predetermined time periodbefore the end of service of the battery unit 210 is reached, an ERIsignal, which may prompt the healthcare provider and/or the patient toschedule elective replacement of an electronic device, may be assertedto provide a warning as necessary (Step 790). ERI is typicallydetermined as simply a predetermined time, for example from 1 week to 1year, more typically 6 months, earlier than EOS. In an alternativeembodiment, the ERI signal may be defined as a particular charge levelremaining (C_(ERI)) above the EOS charge, C_(EOS). In this embodiment,the time period remaining until the ERI signal could be calculated bydividing C_(EOS) by the total depletion rate r_(t) and subtracting theresulting time period from the time to EOS as calculated in equation 6.

The time to EOS provides a warning to the healthcare provider and/orpatient that the energy or charge supply will be depleted very shortly.Therefore, the time to EOS is reported to the implantable medical device110 and/or to the external device 150 (block 740). The ERI is alsoreported to the implantable medical device 110 and/or to the externaldevice 150, which is then brought to the attention of the patient and/ora medical professional.

The accuracy of the estimate of time remaining until EOS or until thegeneration of an ERI may be further refined by adding higher order termsto either of the linear equations 7 or 8. In various embodiments, thenon-linear battery voltage depletion curve may be more accuratelyapproximated by a higher order polynomial equation.

More specifically, the second region 302 of the battery voltagedepletion curve (i.e., after V_(knee)) may be more accuratelyapproximated by a higher order polynomial equation, while the firstregion 300 of the battery depletion curve is approximated by either aconstant (i.e., a linear approximation having a negligible slope), alinear approximation, or in other alternative embodiments, a non-linear,higher order polynomial equation. The equation for time remaining may bethus improved, reflecting the more accurate voltage curve (or portion ofthe voltage curve) of a higher order, and resulting in a more accuratecalculation of the estimate of time remaining until EOS. A polynomialfitting software application employing any curve fitting technique,e.g., a least-squares fit, a LOESS (local polynomial regression fitting)function, or the like, may be run on a data set empirically obtained toapproximate the battery voltage depletion curve with a polynomialequation of at least Nth degree, of the form: f(x)=a₀+a₁x+ . . .+a_(n-1)x^(n-1)+a_(n)x^(n). In one embodiment, the polynomial equationapproximating the battery voltage depletion curve is of at least the2^(nd) degree.

The graphs of FIG. 8 and FIG. 9 further illustrate how the batteryvoltage depletion curve may be more accurately approximated by anon-linear equation. FIG. 8 shows a plot of battery capacity estimationfor a battery having a voltage depletion curve described by a polynomialequation at least in the second region 302 after the transition voltageV_(knee). The battery voltage depletes after V_(knee) as shown for “lowload” (i.e., for a low current drain), and for “high load” (i.e., for ahigh current drain) conditions. FIG. 8 reflects that at least a secondorder polynomial (identified as Poly (Low Load, device driven at lowlevel) and Poly (High Load, device driven at high level) are moreaccurate approximations of the battery voltage than the linear estimate,also shown. For a particular embodiment, specific polynomial equationsfor an illustrative battery such as those used in various IMDs are shownon the graph (one for low load conditions such as shown for Eq. 10, andone for high load conditions such as shown for Eq. 11), and follow here:y=−52.948x ²+11.73x+1.9982; R²=0.9989; and  (Eq. 10)y=−54.027x ²+11.372x+2.0144; R²=0.9971;  (Eq. 11)where x is the battery capacity remaining, y is the battery voltage, andR² is the coefficient of determination (a measure of accuracy to theactual data) for the polynomial approximations.

The polynomial equations provided may be solved to arrive a percentageremaining based on V_(bat). Using the quadratic equation, the generalform of the equation to solve for percentage remaining is:percentage(%)remaining={(−b+SQRT[b ²−4a(c−V_(bat))])/2a};  (Eq. 12)where a and b are the polynomial coefficients of the x² and x terms,respectively, and c is the constant.

While coefficients for the low load could be used, in variousembodiments, it is preferable to include consideration for the curveshape under the high load condition. FIG. 9 provides a comparison of lowload and high load coefficients, including an intermediate polynomialthat represents intermediate coefficients achieved with roundedaverages. The intermediate polynomial provides a capacity estimationbetween the low load and high load conditions.

Calculating the percentage remaining using the intermediate polynomialequations from FIG. 9 yields:percentage(%)remaining={0.107944−[SQRT(561.4025−214*V_(bat))]/107}  (Eq.13)

Note again that this calculation is performed when V_(bat) is less thanthe transition voltage, i.e., less than V_(knee), which for anillustrative battery is 2.6 volts.

The time remaining until EOS (in seconds) can be computed according tothe equation:T_(EOS)=% remaining*(C_(initial)/I_(avg));  (Eq. 14)where C_(initial) is the initial battery charge (in μAsec) and I_(avg)is the average current consumption rate (in μA).

A further refinement to the estimation of time remaining until EOS oruntil the generation of an ERI is to use a variable V_(knee), ratherthan a constant V_(knee) 310. In such an embodiment, V_(knee) is avariable function of, for example, I_(avg), rendering Equation 9 amulti-variable, non-linear equation. V_(knee) 310 is the voltageassociated with the knee, or transition point of non-linearity along thebattery voltage depletion curve. The precise voltage along the curvewhere the actual transition takes place may vary with the programmedsettings and I_(avg). For simplicity's sake, the V_(knee) 310 may bechosen and set as a constant value within the range of possible V_(knee)values for the various programmed settings, determined experimentally.For example, in an embodiment, V_(knee) 310 may be predetermined and setto a constant voltage of 2.6 volts. However, the V_(knee) value 310 usedin the time calculation (Eq. 9) may also be permitted to vary withI_(avg) and program setting changes for a greater degree of accuracy inthe estimate of time remaining until EOS.

Specifically, V_(knee) may vary according to I_(avg) and programsettings as shown in the graph of FIG. 10. Based upon the batterydescribed by the graph of FIG. 10, the V_(knee) (i.e., the voltagedischarge curve inflection point coinciding with approximately 90%depletion) may be seen as variable with load current as shown. Anestimation for this non-linear variability in V_(knee) may be any orderof equation, with the following being an illustrative embodiment:V_(knee)=MAX of 2.6 or (2.85 v−(LOG(I_(avg)/3))/10));  (Eq. 15)wherein MAX is the mathematical function to select the greater of twovalues, V_(knee) is the greater value of either 2.6 volts or theremaining term, and I_(avg) is the estimated average current in μAmps.

In a first embodiment shown in FIG. 10, the I_(avg) is approximately 30μA, as shown by the dashed plot line. In a second embodiment shown inFIG. 10, the I_(avg) is approximately 300 μA, as shown by the solid plotline. As seen, there is a difference of more than 0.1 v for the V_(knee)at approximately 90% depleted in the two plots. Accounting for thenon-linear relationship of V_(knee) to I_(avg) therefore renders thecalculation of time remaining to EOS or ERI more accurate.

In some embodiments, V_(bat) is modified prior to use in EOS or ERIcalculations based on temperature of the battery. In batteries such asat least some of those used in IMDs as disclosed herein, the voltageacross the battery is impacted by extreme temperatures. Prior toimplant, the value for V_(bat) that is reported may adversely affect theestimate of time remaining until EOS. Specifically, the V_(bat) reportedin very cold temperatures, such as those an IMD might be exposed toduring shipping and/or storage, is lower than the true voltage acrossthe battery. As such, the time remaining until EOS calculated based onsuch an inaccurate V_(bat) is a much shorter time than the IMD actuallyhas remaining, which may result in a physician needlessly discarding anIMD prematurely. Thus, shipment or storage in cold temperatures mayresult in IMD units not being used, as thought to be too near the end ofservice, when the V_(bat) is artificially low.

In order to modify V_(bat) to correct for any adverse effects of extremetemperature, the temperature is measured and the V_(bat) value isautomatically corrected by a known amount corresponding to thetemperature. Generally, there is an approximately linear relationshipbetween the temperature and the number of volts necessary to correctV_(bat) for temperature (as shown in the graph of FIG. 11), thus thecorrection factor may be directly related to the measured temperature.Alternatively, the relationship between the temperature and number ofvolts necessary to correct V_(bat) for temperature may be approximatedby a higher order polynomial equation, and as is well known in the art,a higher order equation may be solved to calculate the proper amount ofvoltage to add in order to correct V_(bat).

In a further alternative embodiment, rather than actually correcting theV_(bat) value for temperature, the temperature may be communicated to auser of the external unit 150, such as a physician, to give notice tothe physician or other user of the external unit 150 that thetemperature of the IMD unit should be permitted to stabilize in anenvironment similar in temperature to the environment the IMD is in whenimplanted. By first permitting the IMD's temperature to stabilize, thetime remaining to EOS may be re-evaluated with a more accurate value forV_(bat) once the temperature is no longer extreme.

In one embodiment, the value for V_(bat) may be compensated fortemperature according to the following algorithm:V_(compensated)=V_(measurement)+(K×(T_(baseline)−T_(measurement)));  (Eq.16)wherein V_(compensated) is the battery voltage compensating fortemperature, V_(measurement) is the battery voltage measured, K is thecompensation constant in volts per degree, T_(baseline) is the baselinetemperation for compensation (determined during calibration) andT_(measurement) is the temperature at the time of battery voltagemeasurement. In an illustrative embodiment using a particular model ofLi/CFx battery, the compensation constant K is 1.2 mV/degree Celcius,and T_(baseline) is 37° C. Thus, voltage would be compensatedaccordingly:V_(compensated)=V_(measurement)+(0.0012×(37−T_(measurement))).  (Eq. 17)

A further refinement to the estimation of time remaining until EOS oruntil the generation of an ERI is modifying the V_(bat) prior to use incalculations with a constant factor, depending on where the battery isoperating within its useful life. Specifically, in batteries such asthose used in IMDs 110 such as that of the present disclosure, thebattery may report an artificially low voltage due to the internalbattery impedance that later in the battery's useful life has little tono impact on the reported voltage. The internal battery impedance has animpact on the reported battery voltage typically in the first 2% of thebattery's useful life, but may affect the reported battery voltage forup to around the first 7-8% of the battery's useful life. The IMD 110may alter the V_(bat) value communicated to the external unit 150 formore accurate calculation of the time remaining to EOS, oralternatively, the V_(bat) value used in the external unit 150 may bemodified to more accurately calculate the time remaining to EOS. Thebattery voltage may, in an embodiment in which the V_(bat) value iscorrected in the early portion of its life, be governed by Equation 18.

$\begin{matrix}{\begin{matrix}\; & {V_{bat} + C_{const}} & {{{{For}\mspace{14mu}\left( {V \geq {2.45\mspace{14mu}{volts}}} \right)}\&}\left( {C_{d} \leq {7.5\%}} \right)} \\{V_{bat} =} & \; & \; \\\; & V_{bat} & {{For}\mspace{14mu}\left( {{Vbat} < {2.45\mspace{14mu}{volts}}} \right)\mspace{14mu}{or}\mspace{14mu}\left( {C_{d} > {7.5\%}} \right)}\end{matrix}.} & \left( {{Eq}.\mspace{14mu} 18} \right)\end{matrix}$

The correction factor amount C_(const), introduced by Equation 10,injects a non-linear discontinuity into the battery voltage value,thereby introducing non-linearity into the entire algorithm representedby Equation 9. The constant C_(const) may be set during manufacture,according to calibration and characteristics known about the battery,such as its characteristic internal battery impedance at the beginningof its useful life. In a particular embodiment, in order to correct forthe internal battery impedance inaccuracy in the preferred embodiment,C_(const) is set for at least 0.25 volts, and preferably for 0.5 volts.As can be seen from Equation 18, once the battery is no longer in theearliest portion of its useful life and the effects of the internalbattery impedance on battery voltage have lessened, the value forV_(bat) is generally accurate enough so as to not require adding thecorrection factor.

Alternatively, in an embodiment in which the V_(bat) value is correctedin the early portion of its life, the battery voltage may be governed byEquation 19.

$\begin{matrix}\mspace{506mu} & \left( {{Eq}.\mspace{14mu} 19} \right)\end{matrix}$ $\begin{matrix}\; & {V_{bat} + {R_{const} \times I_{avg}}} & {{{{For}\mspace{14mu}\left( {V \geq {2.45\mspace{14mu}{volts}}} \right)}\&}\left( {C_{d} \leq {7.5\%}} \right)} \\{V_{bat} =} & \; & \; \\\; & V_{bat} & {{For}\mspace{14mu}\left( {{Vbat} < {2.45\mspace{14mu}{volts}}} \right)\mspace{14mu}{or}\mspace{14mu}\left( {C_{d} > {7.5\%}} \right)}\end{matrix}.$wherein the compensation factor for the impedance is not a voltageoffset, as in Equation 18, but is instead a variable based upon a fixedbattery impedance value R_(const) and the average current I_(avg) forthe particular programmed settings.

Turning now to FIG. 12, a more detailed flow chart depicting analternative embodiment of the calculation of the time to the end ofservice (EOS) and/or elective replacement indicator (ERI) signals, asindicated in step 430 of FIG. 4, is illustrated. The method of FIG. 12begins similarly to the method of FIG. 7, and the steps of blocks710-725 are not repeated here, as these steps are carried out asdescribed above. The charge depleted C_(d) is calculated by a methodsubstantially as shown in FIG. 6 and previously described (step 1200).Charge depleted may be used to determine the charge remaining.

Additionally, the voltage across the battery unit is measured (step1210). In one embodiment, the voltage across the battery may be measuredperiodically while in an alternative embodiment, the voltage across thebattery is measured when the IMD 110 is in communication with theexternal unit 150. The temperature of the battery unit is also measured(step 1220), to determine whether a temperature correction is indicatedfor the measured voltage. Additionally, for an embodiment having avariable V_(knee), the variable V_(knee) is calculated based on I_(avg)(step 1230), as discussed above.

The charge depleted calculated in step 1200, the voltage across thebattery measured in step 1210, the temperature measured in step 1220,and the value for V_(knee) (if variable) determined in step 1230 arestored in a memory in the IMD 110 and/or transmitted from the IMD 110 tothe external unit 150 for storage in a memory in the external unit 150(step 1240). The measured voltage is corrected for temperature (step1250) as described above with respect to Eq. 16. In alternateembodiments, this correction may be omitted or made conditional if themagnitude of correction exceeds a predetermined threshold.

A comparison of the compensated battery voltage and the V_(knee) isperformed to see whether the compensated battery voltage is greater thanor equal to the V_(knee) (step 1250). When the compensated voltageacross the battery unit is greater than or equal to the voltageassociated with the knee, the time remaining is calculated by dividingthe remaining charge (based on the charge depleted determined in step1200) by the total depletion rate (step 1280). The time remaining isstored and/or transmitted to the external unit 150 (step 1285) asdescribed above, and an indicator is generated as necessary (step 1290).

Returning to 1260, when the compensated voltage across the battery unitis less than the voltage associated with the knee, the time remaining iscalculated according to a higher order polynomial function of thecompensated voltage, and a voltage associated with the EOS (V_(EOS)312), the initial charge (C_(initial)), and a percentage of batterycapacity typically remaining at the knee (% knee) by the product of theaverage current (I_(avg)) and the different between the voltageassociated with the knee (V_(knee) 310) and the voltage associated withthe EOS (V_(EOS) 312) as described above with respect to Eqs. 13-14(step 1265).

Utilizing embodiments of the present disclosure, a more accurateassessment of the status of the battery may be assessed, therebyproviding better warnings to the user and/or to a healthcare providerassessing the operations of the IMD 110. Various end of service signals(EOS) and/or elective replacement indication (ERI) signals may beprovided to indicate the status of the operation of the IMD 110. In anembodiment, an ERI signal may be generated when there are six monthsleft until EOS, according to the calculations discussed herein.

The particular embodiments disclosed above are illustrative only, as thedisclosure may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. The particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope and spirit of the disclosure. Accordingly, the protection soughtherein is as set forth in the claims below.

What is claimed is:
 1. An implantable medical device comprising: anenergy storage device; a stimulation unit that is operatively coupled tothe energy storage device and configured to provide an electricalsignal; a memory device configured to store first electrical chargedepletion rate information associated with an idle mode and secondelectrical charge depletion rate information associated with astimulation mode; and a controller that is operatively coupled to thememory device, the stimulation unit and the energy storage device,wherein the controller comprises: a charge depletion determination unitconfigured to estimate an amount of energy depleted during operationbased on a first depletion amount and a second depletion amount, whereinthe first depletion amount is calculated based on the first electricalcharge depletion rate information applied to one or more periodsassociated with the idle mode, and wherein the second depletion amountis calculated based on the second electrical charge depletion rateapplied to one or more periods associated with the stimulation mode; avoltage determination unit configured to determine whether a measuredvoltage across the energy storage device is greater than or equal to atransition voltage of the energy storage device; and an energy indicatorunit configured to indicate an amount of energy remaining in the energystorage device, wherein when the measured voltage is greater than orequal to the transition voltage, the amount of energy remaining isdetermined based on the amount of energy depleted, and wherein when themeasured voltage is less than the transition voltage, the amount ofenergy remaining is determined based on a difference between themeasured voltage and a voltage associated with end of service of theenergy storage device.
 2. The implantable medical device of claim 1,wherein when the measured voltage is less than the transition voltage,the amount of energy remaining is determined by dividing a first valueby a second value, wherein the first value is determined based on aproduct of the difference between the measured voltage and the voltageassociated with end of service of the energy storage device, an initialcharge of the energy storage device, and a percentage of capacity of theenergy storage device at the transition voltage, and wherein the secondvalue is determined based on a product of an average current consumptionrate and a difference between the transition voltage and the voltageassociated with the end of service of the energy storage device.
 3. Theimplantable medical device of claim 1, wherein the second electricalcharge depletion rate information is based at least in part on one ormore stimulation parameters related to the stimulation mode.
 4. Theimplantable medical device of claim 3, wherein the one or morestimulation parameters include a pulse width, a pulse frequency, a pulseamplitude, or a combination thereof.
 5. The implantable medical deviceof claim 1, wherein the transition voltage is approximately 2.6 volts.6. The implantable medical device of claim 1, wherein the secondelectrical charge depletion rate information is based at least in parton lead impedance of a lead assembly that couples an electrode assemblyto the stimulation unit.
 7. The implantable medical device of claim 1,wherein the amount of energy depleted is determined based at least inpart on an impedance correction factor.
 8. The implantable medicaldevice of claim 7, wherein the impedance correction factor is a constantimpedance value.
 9. The implantable medical device of claim 1, whereinthe amount of energy depleted is determined based at least in part on atemperature correction factor, and wherein the temperature correctionfactor is based at least in part on a measured temperature of the energystorage device.
 10. The implantable medical device of claim 1, whereinwhen the measured voltage is less than the transition voltage, theamount of energy remaining is determined using linear approximation. 11.The implantable medical device of claim 1, wherein when the measuredvoltage is less than the transition voltage, the amount of energyremaining is determined using polynomial regression.
 12. The implantablemedical device of claim 1, wherein the energy indicator unit is furtherconfigured to calculate time remaining until the energy storage deviceis not operational, and wherein when the measured voltage is less thanthe transition voltage, the time remaining is determined according tothe following equation: time remaining =percent remaining×[C_(initial)/I_(avg)]; wherein percent remaining=(−b+SQRT(b²−4a(c−V_(measured))))/2a; wherein a, b, and c arepredetermined polynomial coefficients; wherein V_(measured) is themeasured voltage; wherein C_(initial) is an initial charge of energystorage device; and wherein I_(avg) is average current consumption rate.13. The implantable medical device of claim 12, wherein the measuredvoltage used to determine the time remaining adjusted based on animpedance correction factor.
 14. The implantable medical device of claim13, wherein the impedance correction factor is a constant value.
 15. Theimplantable medical device of claim 13, wherein the measured voltage isadjusted based on the impedance correction factor according to thefollowing equation: adjusted measured voltage used to calculate timeremaining =V_(measured)+[R_(const)×I_(avg)]; wherein R_(const) is theimpedance correction factor constant.
 16. The implantable medical deviceof claim 12, wherein a compensated voltage (V_(compensated)) isdetermined based on compensation of the V_(measured) by a measuredtemperature of the energy storage device, and wherein the percentageremaining to determine the time remaining is calculated based onV_(compensated) according to the following equation: wherein the percentremaining =(−b+SQRT(b²−4a(c−V_(compensated))))2a.
 17. The implantablemedical device of claim 16, wherein the compensated voltage isdetermined according to the following equation:V_(compensated)=V_(measured)+(K×(T_(baseline)−T_(measurement))); andwherein T_(measured) is the measured temperature; wherein T_(baseline)is a base temperature; and wherein K is a temperature compensationconstant.
 18. The implantable medical device of claim 1, wherein thetransition voltage is determined according to the following equation:transition voltage =maximum of W volts and (X volts−(LOG(I_(avg)/Y))/Z);wherein W is a variable or constant voltage; wherein X is a voltagegreater than W volts; wherein I_(avg) is the average current consumptionrate; and wherein Y and Z are predetermined coefficients.